This invention relates to control systems for the pneumatically powered actuators of valves such as those often used in the pulp and paper, chemical and petroleum industries. More particularly, it relates to a system that provides for controlling the position of a pneumatically operated valve with an electrical control signal.
It is desirable to utilize piezo valves as part of a control system for a pneumatically powered valve actuator because piezo valves provide an extremely long cycle life and reliability, extremely low power requirements, and fast on/off times. However, the volume of air they pass is relatively small compared to the volume required to quickly move a control valve into a new adjusted position. In some industrial applications, a relatively significant force is required to achieve movement of the valve, necessitating a correspondingly large surface area on the part of the actuator exposed to air pressure. Piezo valves are typically unable to provide the sustained volume of air necessary over the surface area of the movable part.
Accordingly, an object of the present invention is to provide a valve control system that utilizes piezo valves to take advantage of their inherently desirable characteristics, yet the volume of air supplied to the valve actuator of the system is boosted to such an extent that the actuator quickly repositions the valve into the desired adjusted position.
In carrying out the foregoing object, the present invention contemplates a system in which multiple piezo valves are subject to an electrical control signal to either initiate or terminate a pilot pneumatic output at the established control pressure. Such pilot output is in turn directed to appropriate valve assemblies of a volume booster circuit which is likewise connected to a source of pressurized gas at the same pressure as that supplied to the piezo valves. Thus, the pneumatic pilot signal from the piezo valves is used to determine the opened or closed state of valve assemblies in the booster mechanism. These booster valve assemblies are capable of passing much greater volumes of gas to the actuators than the small pilot signals produced by the piezo valves. Consequently, the instantaneous action of the piezo valves is obtained, along with sufficiently large volumes of gas to quickly move the operating part of the actuator.
The control system of the present invention utilizes pistons in the booster valve assemblies that operate on the principle of unequal piston areas on opposite operating surfaces thereof. Thus, both surfaces of the piston may be simultaneously exposed to the same operating pressure in the form of a pneumatic signal from the piezo valve and an operating volume from the gas source. However, even though the pressures are the same on both faces of the piston, the piston will be moved in a direction generally toward the smaller surface area because the total force on the piston is greater on the side with the larger surface area. In the preferred form of the invention, the side of the piston exposed to the pneumatic signal from the piezo valve is the side with the large surface area, while the side exposed directly to the gas source is the smaller.
In preferred forms, the control system may be either single acting or double-acting. In a system configured for a single acting actuator, the actuator has only one operating gas chamber on one side of its movable part, while a spring is disposed on the opposite side. In a double-acting version, pressure chambers are located on opposite sides of the movable part of the actuator so that pressurized gas is used to move the part in both of its adjusting directions of movement. In both versions, the control system is capable of moving the actuator in valve opening or valve closing directions, plus holding the actuator in a selected stationary position.
In its preferred form, each booster valve assembly utilizes a piston confined between a pair of diaphragms as the shut-off valve component within the assembly. The piston responds to the pneumatic pilot pressure to push one of the diaphragms into sealing engagement with the valve seat and thus close the operating flow path through the valve assembly. When pilot pressure on the large face side of the piston is absent, the source pressure on the small face side of the piston shifts it away from the diaphragm to permit the diaphragm to uncover and open the flow path.
A second embodiment of the valve assembly uses an integral piston and valve wherein one face of the piston is directly exposed to pilot pressure from the piezo valve and the other face of the piston is both directly exposed to operating pressure from the gas source and has a sealing surface that, when engaged with the valve seat, functions to close the valve assembly.
A third embodiment of the valve assembly uses a piston confined between a ball valve and a diaphragm. The ball valve directly opens and closes the valve seat rather than the piston, although the ball valve is physically moved by the piston as the piston responds to the presence or absence of pressure on its opposite sides.